Circuitry for allocating access to a demand shared bus

摘要

Circuitry is disclosed for allocating requests for demand-sharing bus access among a plurality of service requesting ports. During bus contention time, each requesting port synchronously and sequentially applies the digits of its assigned unique priority code to the bus beginning with the most significant digit. After the application of all digits, only the requesting port having the highest code remains in contention and it seizes the bus. The present invention provides flexibility in port preference by the use of a plurality of status flip-flops in each port for generating dynamic port parameter bits. The generated parameter bits are normally applied to the bus as the most significant bits of a dynamic port priority code during contention time. The state of the status flip-flops is controlled by circuitry which counts the number of packets of a specified size currently stored in the buffer memory of each port. This gives preference to the port whose memory contains the largest number of packets of the specified size. For a PBX, this permits short packets containing time sensitive system control messages to be given preference over longer packets containing relatively non-sensitive customer provided business information such as word processing files.